1. Field of the Invention
The present invention relates to a transfer substrate and a method for fabricating an organic electroluminescent element. More particularly, the present invention relates to a transfer substrate adapted for use in pattern formation of a luminescent layer and also to a method for fabricating an organic electroluminescent element using the transfer substrate.
2. Description of the Related Art
In recent years, attention has been drawn to display devices using organic electroluminescent elements (so-called organic EL element) as a flat panel display device that is lightweight and high in efficiency.
The organic electroluminescent element used for such a display device is provided on a transparent substrate such as, for example, glass and is so arranged that an anode made of ITO (indium tin oxide: transparent electrode), an organic layer and a cathode are laminated in this order as viewed from the substrate side. The organic layer has such an arrangement that a hole injection layer, a hole transport layer and an electron transportable luminescent layer are successively laminated as viewed from the anode side. With the organic electroluminescent element thus arranged, the electrons injected from the cathode and the holes injected from the anode are re-combined at the luminescent layer, whereupon light is taken out from the substrate side through the anode.
A full-color display device using such organic electroluminescent elements is formed such that organic electroluminescent elements of individual colors including R (red), G (green) and B (blue) are arrayed on a substrate. In the manufacture of such a display device, luminescent layers made of organic light-emitting materials capable of emitting the respective colors should be formed in pattern for every luminescent element. The pattern formation of the luminescent layers is now carried out, for example, according to a shadow masking process wherein a luminescent material is vacuum deposited or coated through a mask formed with an opening pattern in a sheet or by an ink jet technique.
In the pattern formation made by the shadow masking process, a difficulty has been involved in further miniaturization and a high degree of integration of organic electroluminescent elements because of a difficulty in further microfabrication of an opening pattern formed in the mask and also of a difficulty in pattern formation of high positional accuracy in a luminescent element region owing to the flexibility and elongation of the mask. Upon contact of the mask formed with an opening pattern therein, functional layers including a previously formed organic layer are liable to break, thereby causing a production yield to be lowered.
The pattern formation by the ink jet process is limited in patterning accuracy, for which a difficulty is involved in microfabrication and a high degree of integration of luminescent elements and also in the formation of a large-sized substrate.
To avoid this, there has been proposed a transfer process (i.e. a heat or thermal transfer process) using an energy source (heat source) as a new pattern forming process of a luminescent layer constituted of an organic material and other functional layers. The display device using the thermal transfer process is manufactured, for example, in the following way. Initially, a lower electrode is formed on a substrate of a display device (hereinafter referred to as “device substrate”). On the other hand, a luminescent layer is formed on another substrate (hereinafter referred to “transfer substrate”) through a light-heat conversion layer. The device substrate and the transfer substrate are arranged so that the luminescent layer and the lower electrode are facing each other, under which a laser beam is irradiated from the transfer substrate side thereby causing the luminescent layer to be thermally transferred onto the lower electrode of the device substrate. At this stage, when the spot irradiated laser beam is scanned, the luminescent layer is thermally transferred only at a given region on the lower electrode in high positional accuracy (see Japanese Patent Laid-open Nos. 2002-110350 and Hei 11-260549).
With respect to an organic electroluminescent element fabricated according to the thermal transfer process, there has been proposed a method of improving a luminescent efficiency and a luminance half-time life by subjecting a device substrate and a transfer substrate to heat treatment prior to heat or thermal transfer of a luminescent layer (see Japanese Patent Laid-open No. 2003-229259).
Further, there has been proposed a method of increasing sensitivity wherein a sublimating compound is used as a donor of high sensitivity of a laser-induction thermal image-forming system (see Japanese Patent Laid-open No. Hei 10-6643).
On the other hand, there has been disclosed a technique wherein a compound having an aromatic ring having four or more condensed polycyclic hydrocarbons is used as a dopant in a luminescent layer of an organic electroluminescent element (see Japanese Patent Laid-open No. Hei 9-241629).
However, the luminescent elements obtained by use of such thermal transfer processes have problems in that the transfer layer degrades by laser irradiation although depending on the types of organic materials used for the transfer layer, so that when compared with a luminescent element made by the shadow mask process, the luminescent efficiency becomes lower, the drive voltage increases, and the luminance half-time life becomes shorter. Thus, the improvements made by such techniques as set out above are not satisfactory with respect to the effects brought about thereby.